It forms a guise, or at least it tries, an uncontrollable, gasping surprise
Poo's Mirror ability
It forms a guise, - A 'guise' is an appearance, which mirror forms.
or at least it tries, - Well, Poo may be highly disciplined, but there are still some things even he can't copy. I once tried mirror on Giygas.. Oh well, it was worth a shot anyway, heheh.
an uncontrollable, gasping surprise - When Poo tries to copy someone, he takes a deep breath. When he does, the narrator (I guess) says: Yikes! Poo transformed into... And once Poo takes on the form of the enemy, he'll be out of your control. Kinda like reidman when he's listening to Five Iron Frenzy ;-)